skge: add dma_mapping check

This old driver never checked for DMA mapping errors.
Causing splats with the new DMA mapping checks:
	WARNING: at lib/dma-debug.c:937 check_unmap+0x47b/0x930()
	skge 0000:01:09.0: DMA-API: device driver failed to check map

Add checks and unwind code.

Reported-by: poma <pomidorabelisima@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>